Electrolytic stripping of copper, zinc and tin based coatings from a ferrous base using an alkaline pyrophosphate electrolyte



July 23, 1968 M. c. BLUME 3.3949

ELECTROLYTIC STRIPPING OF COPPER ZINC AND TIN BASED COATINGS FROM A FERRDUS BASE USING AN ALKALINE PYROPHOSPHATE ELECTROLYTE Filed Oct. 22, 1965 2 Sheets-Sheet 1 3 Z9 Z3 F/6./

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INVENTOR. MATT/IE! C. .54 1/446 M. c. BLUME 3,394,063 ELECTROLYTIC STRIFPING OF COPPER ZINC AND TIN July 23, 1968 BASED COATINGS FROM A FERROUS BASE USING AN ALKALINE PYROPHOSPHATE ELECTROLYTE 2 Sheets-Sheet 2 Filed Oct. 22, 1965 MATT/A514} 6. 540446 INVENTOR.

United States Patent 3,394,063 ELECTROLYTIC STRIPPING OF COPPER, ZINC AND TIN BASED COATINGS FROM A FERROUS BASE USING AN ALKALINE PYROPHOSPI-IATE ELECTROLYTE Matthew C. Blume, 18 Myrtle Ave., Ansonia, Conn. 06401 Filed Oct. 22, 1965, Ser. No. 500,626 14 Claims. (Cl. 204-146) This invention is that of a process of electrolytically stripping coatings of copper, zinc, tin, brass or bronze, or other alloys wherein these individual metals predominate, from ferrous metals, to leave the ferrous metals free of the particular non-ferrous metal coating and provide in a readily freely available state the stripped off nonferrous metal.

More particularly the method of the invention involves stripping said non-ferrous metal (singly or as such an aforesaid alloy) coating from a ferrous metal base or substrate by passing an electric current through the thus coated ferrous metal while it is submerged in an alkaline aqueous bath having a water-soluble pyrophosphate dissolved in it, through which bath the current passes to a cathode inert to the bath. Such bath is prepared free of any electrolyte which forms a water-soluble salt of the non-ferrous metal and thus would enable its direct electrodeposition as a strongly adherent electroplate.

The expression ferrous metal is used in its generally recognized sense as covering steel (including stainless steel), and iron and its alloys wherein it ordinarily predominates. So also the non-ferrous metals copper, zinc and tin are respectively separately intended not necessarily to be restricted merely to the individual metal itself but to include it as well as the various alloys in which such individual metal is the recognized predominating constituent.

The method of the invention is applicable to remove the non-ferrous metal coating from the ferrous metal substrate regardless of whether the coating was applied as an electroplate, or by spray (as in metallizing), hot dip, or by rolling.

As the invention initially is applicable especially in obtaining clean steel scrap from more generally scrap copper or brass plated, and sometimes bronze plated, sheet steel, the invention conveniently can be explained more fully by describing that specific application of it.

Sheet steel plate with copper or brass are used very extensively, and bronze plated to a lesser extent, in producing articles such as lipstick and cigaret holders, compact cases, and others, as punchings. The waste skeleton of the plated sheet left from the punching operation, along with any rejected holders and cases, has no value and is given to the scrap dealers free merely for carting it away, with other saleable scrap. The copper or brass or bronze coated scrap is of no value to the scrap dealer because the steel mills cannot use it due to the deleterious effect of high copper content in steel.

No suitable commercially practical operable or economical method of treating such non-ferrous metal coated ferrous metal scrap is available to enable obtaining its iron in a form in which it can be used, and for the scrap so to become valuable.

A valuable feature of this invention then is that it enables easy removal of the non-ferrous coating to leave the ferrous metal without loss in a clean condition for it to be sold as prime scrap.

A further feature of the invention is that its operating cost is low and it requires little labor attention except at loading and unloading.

3,394,063 Patented July 23, 1968 'ice Another feature of the invention is the low cost of operation of the bath for the pyrophosphate electrolyte is not consumed in use, and loss by dragout can be kept exceedingly low.

Still a further feature of the invention is the ready recovery of the stripped off copper or other non-ferrous coating metal in a form in which it is readily saleable at good value.

Yet another feature of the process is the stable character of the electrolysis bath which required no adjustment during operation.

It is a further feature of the process that the electrolysis stops when all of the copper coating has been removed so that there is no loss in iron and no chance of contaminating the recoverable non-ferrous metal with iron.

Considered broadly, the invention is that of a process of removing a non-ferrous metal coating from a ferrous metal substrate, which method comprises submerging the non-ferrous metal coated ferrous metal in an aqueous alkaline bath having dissolved in it a readily watersoluble pyrophosphate, and passing an electric current through the thus coated submerged ferrous metal as the anode, into and through the bath and to a cathode which is inert to the alkaline bath, and until substantially all of the non-ferrous metal coating has been removed from the ferrous metal substrate. It is advantageous that the bath be free of electrolytes which solubilize copper and allow it to be deposited on a cathode as a continuous and adherent electroplate.

It is a striking feature of the invention that when all of the non-ferrous metal coating is removed, none of the ferrous metal substrate appears to be removed on continuing the flow of current.

The process will operate at a concentration of as little as one quarter of a percent or less of the tetravalent pyrophosphate dissolved in the bath, but at that level the time to reach complete removal of the coating is quite long. Time production improvement is seen in building up to a pyrophosphate anion concentration of 0.75 percent. However, it is advantageous to operate with the pyrophosphate anion concentration at from about 1.5 to about 5.5 percent. The pyrophosphate anion is the tetravalent anion P 0 It is beneficial to provide that anion by dissolving an alkali metal or ammonium pyrophosphate in the bath, and advantageously by using tetra-potassium pyrophosphate (i.e., K P O or tetra-sodium pyrophosphate (i.e., Na P O or mixtures thereof. So far as presently indicated, the potassium salt appears to perform better than the sodium salt. The process will operate with any of these dissolved even to saturation, but nothing is to be gained that high.

At the high levels approaching saturation, the viscosity increases and dragout is greater although it is kept at a minimum by rinsing the cleaned ferrous metal as it is held above the 'bath when removed from it. It is beneficially practical not to use the water-soluble pyrophosphate in a concentration above that which will give greater than about 12 percent of the pyrophosphate anion, although as stated the process will operate at concentrations beyond that.

It is possible to operate the method with the bath at pH 9 and up to pH 9.5. However, above that it is advantageous in that the rate of removal of the non-ferrous coating is increased to about five-fold by maintaining the bath at a pH of from about 10 to about 10.8. Thus, it is advantageous to operate at a pH from about 10 to about 11. The coating removal rate begins to drop when the pH is at about 11.2 and at above about pH 11.5 there does not appear to be any noticeable stripping.

These pH levels are by pH meter. The pH can be adjusted to the desired level, in preparing the bath, by adding sufficient strong alkali hydroxide such as potassium or sodium hydroxide.

While the process operates at ambient temperature, it should be operated at elevated temperature beneficially between about 130 F. to about 190 F., and so far as presently indicated, advantageously in the neighborhood of about 160 F.

As to voltage, it is beneficial, so far as presently indicated, to operate at below about 7 volts and advantageously at from about 3 to about volts. So far as presently indicated, it is beneficial for the current density to be under about 2 amperes per square foot and advantageous for it to be under one ampere per square foot. Much depends on the variation in size of the coated articles to be stripped, as well as the various different sizes of the pieces in the scrap loads. In a number of the best production runs the current density appeared to be between about 0.15 and 0.1 ampere per square foot.

The cathode can be any suitable conductor. Generally it is advantageous to conduct the process in an iron tank. Such tank then can serve as the cathode as can also any other metal tank which is inert to the alkaline bath under the operating conditions and is a conductor.

During the operation of the process, the passage of the current removes the copper or other non-ferrous metal from the coated steel or other ferrous metal; and the copper only loosely adheres to the cathode. The thus loosely adhering copper readily is removed from the cathode by light scraping, and generally where it can be done merely by lightly striking the cathode.

The thus removed copper can be allowed to accumulate at the bottom of the bath tank with any amount of it which fell to the bottom during the operation, or was accumulated there from the prior run or runsto be removed in suitable manner when desired to when it is nearly so deep as soon to interfere with the movement of any part of the apparatus rotated in the tank.

The aqueous alkaline bath can be used, as indicated, quite indefinitely for the water-soluble pyrophosphate is not destroyed in use and dragout losses are insignificant. That is so because they are kept at a minimum by rinsing the coatingcleaned steel with water, as mentioned above, and allowing the rinsings to run back into the tank. Thus, only insignificantly small amounts of water-soluble pyrophosphate need be added as indicated by the liquid level of the tank.

In carrying out the stripping process of the invention, it is advantageous to load the scrap to be stripped of non-ferrous metal coating into a perforated tumbling barrel or drum mounted for rotation on an axially positioned electric conductor metal shaft serving as the anode. The latter is contacted by the scrap while the barrel is submerged in the tank and rotated about the shaft anode, the outer ends of which are insulated from the tank which serves as the cathode. Such practical operation is exemplified "by the apparatus arrangement shown in the accompanying drawings wherein:

FIG. 1 is a longitudinal side elevation of the apparatus, with part of a tank wall in broken section exposing the tumbling barrel (in this case as two units) and its shaft and supports, etc.;

FIG. 2 is a top plan view;

FIG. 3 is a front to rear vertically transverse section on an enlarged scale, along the line 33 of FIG. 1 and viewed in the direction shown by the arrows;

FIG. 4 is a fragmentary foreshortened longitudinally vertical sectional view through the dual barrels and their supports along the line 4-4 of FIG. 3 and viewed in the direction shown by the arrows; and

FIG. 5 is a fragmentary partial sectional view through the brushes encircling the shaft and at the end of the conductor, along the line 55 of FIG. 4 as viewed as directed by the arrows.

The drawings show the steel, bath-holding tank 11, in which the twin or dual perforated wall drums or barrels 12 and 12a are mounted in tandem for rotation about their common steel shaft 14 supported at its outer ends in opposed nylon insulator bearings 15. These bearings are carried in vertical suspension arms 16 depending into the tank with their lower ends spaced upwardly away from the tank bottom.

The upper ends of these arms 16 are secured respectively to the opposed ends of a pair of parallel horizontal channel iron carriers 18. The latter extend over the length of tank 11 and are supported spaced upwardly away from contact with it by a pair of parallel widely horizontally spaced apart hoistengagernent rods 19, each of which extends through both of carriers 18 and has its outer ends seated in the grooves in the top of its respective pair of opposed insulator-separators 20 resting on encircling flange 21 of tank 11.

Motor 23 drives shaft 14 by chain belt 24 riding over motor sprocket 29 and shaft sprocket 30. Current from a power source and rectifier (both not shown) to conductor 25 enclosed in its insulator-sheath 26 flows through the pressure-spring clamped brushes 28 to anode shaft 14. Shaft sprocket 30 is insulated from shaft 14 by centrally interposed nylon insulator rings 32 encircling the shaft and fixed to rotate with its annularly-encircling rest of sprocket 30.

Likewise, each of the end walls of each of the dual drums 12 and 12a is insulated from shaft 14 by a similar centrally interposed nylon insulator ring 33, 33a, and 34 and 340. Each of the drums can be constructed of suitably strong material for the load to be carried in them, for example, sheet steel (which may be stainless) or other suitable metal, strong enough or suitably strengthened non-conductor material. They can be cylindrical or advantageously polygonal, e.g., octagonal (as in FIG. 3).

The peripheral walls of the drums are perforated advantageously all over (by perforations 35), and should permit access for loading and unloading by suitable doors 36 mounted on scabbard hinges and locked by easily closeable and openable catches such as wing-nut tightenable hinged swing-bolts to allow quick unlatching and removal (if desired) of the doors for unloading the drums and also quick replacement and locking of the doors after loading.

As already indicated, steel tank 14 serves as the cathode, and a (negative) conductor 37 leads away from it to complete the circuit. With drums 12 and 12a positioned as seen in FIG. 3, the entire treating assembly including them, shaft 14, its suspension arms 16 can be lifted out of tank 11 by engaging hooks from hoist chains under the two parts of each of hoist-engagement rods 19 just outside of both channel iron carriers 18. The entire assembly thus lifted out of tank 11 then can be let down with the lower ends of suspension arms 16 to rest on the plant floor. If the drums are of such capacity that the load on the shaft may be too great, an intermediate suspension arm 16 with a nylon insulating bearing 15 can be included midway between the drums. Then shaft sprocket 30 and brushes 28 are to be set sufficiently further apart.

With the entire treating assembly thus out of tank 11, the doors of drums 12 and 12a can be removed to allow loading coated scrap into them. A heater (not shown), insulated against receiving current from shaft 14 through the solution used in the tank, is placed conveniently effectively located in tank 14 to enable heating the solution to the selected operating temperature and to maintain it there.

A fairly practical size apparatus as seen in the drawings has a tank about 15 feet along by 5 feet high by 3.5 to 4 feet wide, with each drum about 7 feet long and 25 inches in diameter with adequate clearance on all sides) with a capacity to handle at least 1500 to about 3000 pounds of scrap per drum per run.

The method of the invention and operation of apparatus are illustrated by, but not limited to, the following example: l

800 pounds of anhydrous tetrapotassium pyrophosphate (K P O are chargedinto a tank 14 of the foregoingdimensions (width 3.5 feet), and dissolved by pumping in 1500 gallons of water (providing a solution of very nearly 6% concentration) while the heat is on to raise its temperature to about 160 F.The pH of the solution is adjusted to 10.5 (by meter). In the meantime a ton of scrap copper-coated steel was loaded into each of the dual drums of the apparatus (as shown in the drawings) and having the above-noted dimensions.

The entire drum and carrier assembly is raised and set in place with the drums submerged below the water level in the tank. The electrolysis current is turned on to pass a current of about 120 amperes at a voltage of 4.5 through the bath while motor 23 rotates the drums 12 and 12a at r.p.m. The copper is seen to be completely stripped from the steel an hour and a half and up to two hours (some variation occurs according to the thickness of the non-ferrous coating).

The stripped off copper deposits loosely adherently on the walls of tank 14 and drops down as a finely divided metal sludge over the bottom of the tank on striking its sides. No further metal is removed after the copper thus is stripped off. The electrolysis current then is shut off, the entire assembly of drums and carriers raised out of the bath and held above it while a water spray rinses dragout solution from the drums, their scrap content, the shaft and suspension arms 16 back into the bath beneficially to raise its level back to where it was before the assembly was submerged in it.

The assembly then is moved aside and lowered to the plant floor level, and the cleaned scrap is removed and replaced by a new load of coated scrap, with which the operation of the example is repeated. Such operation is repeated again and again. Instead of copper-coated scrap, the operation can be repeated with brass-coated scrap, or with any other of the hereinabove non-ferrous alloy coated scraps. However, before changing to strip scrap with a different coating, the copper-containing sludge should be removed from the bottom of the tank, and similarly, also with the accumulations of each respectively different non-ferrous metal scrap. Each of these can be rinsed and dried and sold.

The operation conditions can be varied within the ranges disclosed further above as the character of the different types of scrap may require. The K P O may be replaced in part or as a whole by another applicable pyrophosphate, as disclosed hereinabove, for example, tetrasodium pyrophosphate. The pH can be checked from time to time and be adjusted with alkali metal hydroxide when necessary. Small amounts of the desired pyrophosphate can be added when, after many runs, it may be seen to e rlleeded to bring its concentration back to a desired eve Two complete assemblies can be used so that a loaded one can be lowered to submerge its drums in the bath when a finished rinsed one is removed to be unloaded and refilled. It is also helpful to have a wider bath and to have two separate sets of drums submerged and at different stages of completion while a third one is outside being emptied and re-filled.

The process can operate also on lacquered non-ferrous metal coatings and result in removing the lacquer and the metal coating without prior treatment of the lacquerand works similarly with oily and greasy non-ferrous metal coatings.

The tank can be made of steel, or any other conductor metal, or alloys of them, so long as it is an electrical conductor, and advantageously a structural metal higher than copper in the electromotive series and inert to the alkaline electrolysis conditions.

The perforated walls of the tumbler drum can be made of some other suitably strong enough metal or non-metal so long as it is inert to the alkaline electrolysis bath.

While the invention has been explained by detailed description of certain specific embodiments of it, it is understood that various modifications or substitutions can 'be made in any of them within the scope of the appended claims which are intended also to cover equivalents of the specific embodiments.

What is claimed is:

1. The process of stripping from a ferrous metal substrate a non-ferrous metal coating member of the class consisting of copper, zinc, tin, brass, bronze, and other alloys wherein these separate metals predominate; which process comprises:

(a) submerging said non-ferrous metal coated ferrous metal in an aqueous alkaline electrolysis bath having a pH of from 9 to about 11.5 and whose principal electrolyte consists essentially of a pyrophosphate soluble in water to give an alkaline solution and a tetravalent pyrophosphate anion concentration of at least about one-quarter percent by Weight, said solution being free of any solute (i) which can form a water-soluble salt of the non-ferrous metal and enable its electrodeposition as a strongly adherent electrodeposit, and (ii) which can enable any significant deposition of iron, under the operating conditions;

(b) passing an electric current through said coated metal into and through said bath to a cathode inert to said bath under the operating conditions; and

(c) until said non-ferrous metal is substantially completely stripped from the ferrous metal substrate.

2. The process as claimed in claim 1, wherein the pyrophosphate is a member of the class consisting of an alkali metal pyrophosphate and ammonium pyrophosphate.

3. The process as claimed in claim 2, wherein the voltage is under about 7.

4. The process as claimed in claim 2, wherein the voltage is from about 3 to about 5 volts.

5. The process as claimed in claim 2, wherein the current density is under about 2 amperes per square foot.

6. The process as claimed in claim 5, wherein the current density is under about one-half ampere per square foot.

7. The process as claimed in claim 1, wherein the pyrophosphate is an alkali metal pyrophosphate.

8. The process as claimed in claim 7, wherein the pH of the bath exceeds 9.5.

9. The process as claimed in claim 8, wherein the pH of the bath is from about 10 to about 11.

10. The process as claimed in claim 7, wherein the pH of the bath is from about 10 to about 11 and the pyrophosphate concentration is from about 1.5 to about 5.5 percent.

11. The process as claimed in claim 10, wherein the pyrophosphate is at least one of potassium pyrophosphate and sodium pyrophosphate; and removal of metal from the anode discontinues when the non-ferrous metal coating is substantially completely removed from it.

12. The process as claimed in claim 11, wherein the non-ferrous metal coating is one wherein copper predominates; and the copper deposits only loosely adhering to the cathode and is readily removable from it even by merely striking the cathode.

13. The process as claimed in claim 1, wherein the electrolysis bath is contained within confining walls which are an electric conductor and serve as the cathode, and a rigid anode is mounted for rotation about its own axis within said bath and at an angle of less than about 45 degrees to horizontal; and said coated ferrous metal consists of a plurality of pieces of it which are enclosed within perforated continuous confining surfaces encircling said anode as a shaft and rotatable with it; and said anode is so positioned to be contacted by pieces of said ferrous metal and for them to be submerged in the bath; and said ferrous metal pieces are tumbled about within said confining surfaces until the removal of the non-ferrous metal is substantially complete and the so retained ferrous metal pieces stripped of the non-ferrous metal coating are removed within said confining surfaces from said bath.

14. The process as claimed in claim 1, wherein the Water-soluble pyrophosphate is the only solute in the bath.

8 References Cited UNITED STATES PATENTS ROBERT K. MIHALEK, Primary Examiner. 

1. THE PROCESS OF STRIPPING FROM A FERROUS METAL SUBSTRATE OF NON-FERROUS METAL COATING MEMBER OF THE CLASS CONSISTING OF COPPER, ZINC, TIN, BRASS, BRONZE, AND OTHER ALLOYS WHEREIN THESE SEPARATE METALS PREDOMINATE; WHICH PROCESS COMPRISES: (A) SUBMERGING SAID NON-FERROUS METAL COATED FERROUS METAL IN AN AQUEOUS ALKALINE ELECTROLYSIS BATH HAVING A PH OF FROM 9 TO ABOUT 11.5 AND WHOSE PRINCIPAL ELECTROLYTE CONSISTS ESSENTIALLY OF A PYROPHOSPHATE SOLUBLE IN WATER TO FIVE AN ALKALINE SOLUTION AND A TETRAVALENT PYROPHOSPHATE ANION CONCENTRATION OF AT LEAST ABOUT ONE-QUARTER PERCENT BY WEIGHT, SAID SOLUTION BEING FREE OF ANY SOLUTE (I) WHICH CAN FROM A WATER-SOLUBLE SALT OF THE NON-FERROUS METAL AND ENABLE ITS ELECTRODEPOSITION AS A STRONGLY ADHERENT ELEC- 